The present invention relates to an optical system for image transmission, particularly for projection devices of the “head-mounted” type.
The projection devices of the “head-mounted” type have the purpose of presenting an image, for example generated by a microdisplay, superimposed to a background, so that an observer can see the image generated by the microdisplay without distracting his own attention from the background.
Some solutions are already commercially available which present to an observer the image generated by a microdisplay.
The MicroOptical Corp. proposes the solution diagrammatically depicted in FIG. 1 and protected for example by the patents U.S. Pat. No. 5,715,377, U.S. Pat. No. 5,886,822, U.S. Pat. No. 6,023,372, U.S. Pat. No. 6,091,546 and U.S. Pat. No. 6,618,099.
In this solution, the image shown by the microdisplay is projected by means of a light guide within the user's field of view. The input and output surfaces of the guide are substantially spherical and therefore endowed with optical power, and the propagation therein does not occur by internal total reflection on the side surfaces of the guide, but by simple direct propagation: therefore said guide substantially acts as a simple lens capable of focusing the microdisplay in a virtual image placed at the desired distance.
The choice of the direct propagation within the light guide forms the main constraint of the optical system described: in fact, in this configuration, each reflection on the walls of the light guide can originate spurious images which degrade the quality of the presented image. Therefore, the image projection with a resolution higher than VGA (640×480 pixels) within fields of view higher than 16° requires the use of light guides with side walls very large and therefore cumbersome.
FR 2.638.242 (Thomson-CSF) describes a system according to the pre-characterizing part of the appended claim 1. In such a system (see FIG. 2), the propagation occurs by internal total reflection within the light guide, with a consequent decrease of the thickness with respect to the MicroOptical solution described above. Coupling and decoupling into the light guide takes place by means of two partly or totally reflecting parabolic surfaces, placed at the ends of the light guide. Although more compact with respect to the preceding one, this optical solution suffers from a substantial limitation of the field of view presented to the user with values lower than 10°, due to the aberrations resulting from the use of greatly off-axis portions of the parabolic surfaces.
U.S. Pat. No. 5,701,202 (Olympus Optical Co., Ltd.) describes an optical solution, diagrammatically depicted in FIG. 3, based on the use of a prism formed by complex, asymmetrical and non rotational surfaces. The image shown by the microdisplay is coupled and decoupled into the prism according to the optical path shown in the figure, and each ray undergoes a series of reflections and refractions on the complex surfaces which focus a virtual image projected within the user's field of view. The see-through configuration is reached through the addition of an optical element which compensates the distortions introduced by the complex surfaces.
In the disclosed configuration the coupling of the light rays into the prism occurs by direct propagation (as in the MicroOptical configuration) and not by reflection (as in the Thomson configuration): as it can be seen in FIG. 3, due to this choice, the prism must have a length approximately twice the thickness. It follows that, to reach a compact configuration and see-through features, there are only two alternatives:
1) a microdisplay worn on the user's eyes (as in the case depicted in FIG. 3) with a consequent increase of the total front bulkiness of the system;
2) a microdisplay placed in the temporal zone (as in the MicroOptical configuration) with a consequent increase of the thickness of the light guide up to at least 4 cm, in order to allow the image delivery to the user's eye.
In both the configurations, the bulkiness seems to be excessive.
U.S. Pat. No. 6,657,602 (Minolta Co., Ltd.) describes a binocular optical system based on the use of prisms with asymmetrical and non rotational complex surfaces (as in the case of the Olympus patent described above), wherein the residual aberrations are compensated through a pre-distortion of the image shown by the microdisplay. Also in this case, the image rays encounter the prism surfaces which, by means of consecutive reflections and refractions according to what approximately depicted in FIG. 4, focus and project a virtual image within the user's field of view. This configuration, however, presents the same drawbacks of the configuration described above, not being suitable to obtain optical systems with a reduced bulkiness.